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9419eeef | 1 | <chapter name="Particle Decays"> |
2 | ||
3 | <h2>Particle Decays</h2> | |
4 | ||
5 | The <code>ParticleDecays</code> class performs the sequential decays of | |
6 | all unstable hadrons produced in the string fragmentation stage, | |
7 | i.e. up to and including <ei>b</ei> hadrons and their decay products, | |
8 | such as the <ei>tau</ei> lepton. It is not to be used for the decay of | |
9 | more massive <aloc href="ResonanceDecays">resonances</aloc>, such as top, | |
10 | <ei>Z^0</ei> or SUSY, where decays must be performed already at the | |
11 | <code>ProcessLevel</code> of the event generation. | |
12 | ||
13 | <p/> | |
14 | The decay description essentially copies the one present in | |
15 | PYTHIA since many years, but with some improvements, e.g. in the decay | |
16 | tables and the number of decay models available. Some issues may need | |
17 | further polishing. | |
18 | ||
19 | <h3>Variables determining whether a particle decays</h3> | |
20 | ||
21 | Before a particle is actually decayed, a number of checks are made. | |
22 | ||
23 | <p/> | |
24 | (i) Decay modes must have been defined for the particle kind; | |
25 | tested by the <code>canDecay()</code> method of <code>Event</code> | |
26 | (and <code>ParticleData</code>). | |
27 | ||
28 | <p/> | |
29 | (ii) The main switch for allowing this particle kind to decay must | |
30 | be on; tested by the <code>mayDecay()</code> method of <code>Event</code> | |
31 | (and <code>ParticleData</code>). | |
32 | ||
33 | <p/> | |
34 | (iii) Particles may be requested to have a nominal proper lifetime | |
35 | <ei>tau0</ei> below a threshold. | |
36 | ||
37 | <flag name="ParticleDecays:limitTau0" default="off"> | |
38 | When on, only particles with <ei>tau0 < tau0Max</ei> are decayed. | |
39 | </flag> | |
40 | ||
41 | <parm name="ParticleDecays:tau0Max" default="10." min="0."> | |
42 | The above <ei>tau0Max</ei>, expressed in mm/c. | |
43 | </parm> | |
44 | ||
45 | <p/> | |
46 | (iv) Particles may be requested to have an actual proper lifetime | |
47 | <ei>tau</ei> below a threshold. | |
48 | ||
49 | <flag name="ParticleDecays:limitTau" default="off"> | |
50 | When on, only particles with <ei>tau < tauMax</ei> are decayed. | |
51 | </flag> | |
52 | ||
53 | <parm name="ParticleDecays:tauMax" default="10." min="0."> | |
54 | The above <ei>tauMax</ei>, expressed in mm/c.<br/> | |
55 | In order for this and the subsequent tests to work, a <ei>tau</ei> | |
56 | is selected and stored for each particle, whether in the end it | |
57 | decays or not. (If each test would use a different temporary | |
58 | <ei>tau</ei> it would lead to inconsistencies.) | |
59 | </parm> | |
60 | ||
61 | <p/> | |
62 | (v) Particles may be requested to decay within a given distance | |
63 | of the origin. | |
64 | ||
65 | <flag name="ParticleDecays:limitRadius" default="off"> | |
66 | When on, only particles with a decay within a radius <ei>r < rMax</ei> | |
67 | are decayed. There is assumed to be no magnetic field or other | |
68 | detector effects. | |
69 | </flag> | |
70 | ||
71 | <parm name="ParticleDecays:rMax" default="10." min="0."> | |
72 | The above <ei>rMax</ei>, expressed in mm. | |
73 | </parm> | |
74 | ||
75 | <p/> | |
76 | (vi) Particles may be requested to decay within a given cylidrical | |
77 | volume around the origin. | |
78 | ||
79 | <flag name="ParticleDecays:limitCylinder" default="off"> | |
80 | When on, only particles with a decay within a volume limited by | |
81 | <ei>rho = sqrt(x^2 + y^2) < xyMax</ei> and <ei>|z| < zMax</ei> | |
82 | are decayed. There is assumed to be no magnetic field or other | |
83 | detector effects. | |
84 | </flag> | |
85 | ||
86 | <parm name="ParticleDecays:xyMax" default="10." min="0."> | |
87 | The above <ei>xyMax</ei>, expressed in mm. | |
88 | </parm> | |
89 | ||
90 | <parm name="ParticleDecays:zMax" default="10." min="0."> | |
91 | The above <ei>zMax</ei>, expressed in mm. | |
92 | </parm> | |
93 | ||
94 | <h3>Mixing</h3> | |
95 | ||
96 | <flag name="ParticleDecays:mixB" default="on"> | |
97 | Allow or not <ei>B^0 - B^0bar</ei> and <ei>B_s^0 - B_s^0bar</ei> mixing. | |
98 | </flag> | |
99 | ||
100 | <parm name="ParticleDecays:xBdMix" default="0.776" min="0.74" max="0.81"> | |
101 | The mixing parameter <ei>x_d = Delta(m_B^0)/Gamma_B^0</ei> in the | |
102 | <ei>B^0 - B^0bar</ei> system. (Default from RPP2006.) | |
103 | </parm> | |
104 | ||
105 | <parm name="ParticleDecays:xBsMix" default="26.05" min="22.0" max="30.0"> | |
106 | The mixing parameter <ei>x_s = Delta(m_B_s^0)/Gamma_B_s^0</ei> in the | |
107 | <ei>B_s^0 - B_s^0bar</ei> system. (Delta-m from CDF hep-ex-0609040, | |
108 | Gamma from RPP2006.) | |
109 | </parm> | |
110 | ||
111 | <h3>Other variables</h3> | |
112 | ||
113 | <parm name="ParticleDecays:mSafety" default="0.0005" min="0." max="0.01"> | |
114 | Minimum mass difference required between the decaying mother mass | |
115 | and the sum of the daughter masses, kept as a safety margin to avoid | |
116 | numerical problems in the decay generation. | |
117 | </parm> | |
118 | ||
119 | <parm name="ParticleDecays:sigmaSoft" default="0.5" min="0.2" max="2."> | |
120 | In semileptonic decays to more than one hadron, such as | |
121 | <ei>B -> nu l D pi</ei>, decay products after the first three are | |
122 | dampened in momentum by an explicit weight factor | |
123 | <ei>exp(-p^2/sigmaSoft^2)</ei>, where <ei>p</ei> is the | |
124 | three-momentum in the rest frame of the decaying particle. | |
125 | This takes into account that such further particles come from the | |
126 | fragmentation of the spectator parton and thus should be soft. | |
127 | </parm> | |
128 | ||
129 | <p/> | |
130 | When a decay mode is defined in terms of a partonic content, a random | |
131 | multiplicity (and a random flavour set) of hadrons is to be picked, | |
132 | especially for some charm and bottom decays. This is done according to | |
133 | a Poissonian distribution, for <ei>n_p</ei> normal particles and | |
134 | <ei>n_q</ei> quarks the average value is chosen as | |
135 | <eq> | |
136 | n_p/ 2 + n_q/4 + multIncrease * ln ( mDiff / multRefMass) | |
137 | </eq> | |
138 | with <ei>mDiff</ei> the difference between the decaying particle mass | |
139 | and the sum of the normal-particle masses and the constituent quark masses. | |
140 | For gluonic systems <ei>multGoffset</ei> offers and optional additional | |
141 | term to the multiplicity. The lowest possible multiplicity is | |
142 | <ei>n_p + n_q/2</ei> (but at least 2) and the highest possible 10. | |
143 | If the picked hadrons have a summed mass above that of the mother a | |
144 | new try is made, including a new multiplicity. These constraints | |
145 | imply that the actual average multiplicity does not quite agree with | |
146 | the formula above. | |
147 | ||
148 | <parm name="ParticleDecays:multIncrease" default="4.5" min="3." max="6."> | |
149 | The above <ei>multIncrease</ei> parameter. | |
150 | </parm> | |
151 | ||
152 | <parm name="ParticleDecays:multRefMass" default="0.7"min="0.2" max="2.0"> | |
153 | The above <ei>multRefMass</ei> parameter. | |
154 | </parm> | |
155 | ||
156 | <parm name="ParticleDecays:multGoffset" default="0.5" min="0.0" max="2.0"> | |
157 | The above <ei>multGoffset</ei> parameter. | |
158 | </parm> | |
159 | ||
160 | <parm name="ParticleDecays:colRearrange" default="0.5" min="0." max="1.0"> | |
161 | When a decay is given as a list of four partons to be turned into | |
162 | hadrons (primarily for modes 41 - 80) it is assumed that they are | |
163 | listed in pairs, as a first and a second colour singlet, which could | |
164 | give rise to separate sets of hadrons. Here <ei>colRearrange</ei> is | |
165 | the probability that this original assignment is not respected, and | |
166 | default corresponds to no memory of this original colour topology. | |
167 | </parm> | |
168 | ||
169 | <flag name="ParticleDecays:FSRinDecays" default="true"> | |
170 | When a particle decays to <ei>q qbar</ei>, <ei>g g</ei>, <ei>g g g</ei> | |
171 | or <ei>gamma g g</ei>, with <code>meMode > 90</code>, allow or not a | |
172 | shower to develop from it, before the partonic system is hadronized. | |
173 | (The typical example is <ei>Upsilon</ei> decay.) | |
174 | </flag> | |
175 | ||
176 | In addition, some variables defined for string fragmentation and for | |
177 | flavour production are used also here. | |
178 | ||
179 | <h3>Modes for Matrix Element Processing</h3> | |
180 | ||
181 | Some decays can be treated better than what pure phase space allows, | |
182 | by reweighting with appropriate matrix elements. In others a partonic | |
183 | content has to be converted to a set of hadrons. The presence of such | |
184 | corrections is signalled by a nonvanishing <code>meMode()</code> value | |
185 | for a decay mode in the <aloc href="ParticleDataScheme">particle | |
186 | data table</aloc>. The list of allowed possibilities almost agrees with the | |
187 | PYTHIA 6 ones, but several obsolete choices have been removed, | |
188 | a few new introduced, and most have been moved for better consistency. | |
189 | Here is the list of currently allowed <code>meMode()</code> codes: | |
190 | <ul> | |
191 | <li> 0 : pure phase space of produced particles ("default"); | |
192 | input of partons is allowed and then the partonic content is | |
193 | converted into the minimal number of hadrons (i.e. one per | |
194 | parton pair, but at least two particles in total)</li> | |
195 | <li> 1 : <ei>omega</ei> and <ei>phi -> pi+ pi- pi0</ei></li> | |
196 | <li> 2 : polarization in <ei>V -> PS + PS</ei> (<ei>V</ei> = vector, | |
197 | <ei>PS</ei> = pseudoscalar), when <ei>V</ei> is produced by | |
198 | <ei>PS -> PS + V</ei> or <ei>PS -> gamma + V</ei></li> | |
199 | <li> 11 : Dalitz decay into one particle, in addition to the | |
200 | lepton pair (also allowed to specify a quark-antiquark pair that | |
201 | should collapse to a single hadron)</li> | |
202 | <li> 12 : Dalitz decay into two or more particles in addition | |
203 | to the lepton pair</li> | |
204 | <li> 13 : double Dalitz decay into two lepton pairs</li> | |
205 | <li> 21 : decay to phase space, but weight up <ei>neutrino_tau</ei> spectrum | |
206 | in <ei>tau</ei> decay</li> | |
207 | <li> 22 : weak decay; if there is a quark spectator system it collapses to | |
208 | one hadron; for leptonic/semileptonic decays the <ei>V-A</ei> matrix element | |
209 | is used, for hadronic decays simple phase space</li> | |
210 | <li> 23 : as 22, but require at least three particles in decay</li> | |
211 | <li> 31 : decays of type B -> gamma X, very primitive simulation where | |
212 | X is given in terms of its flavour content, the X multiplicity is picked | |
213 | according to a geometrical distribution with average number 2, and | |
214 | the photon energy spectrum is weighted up relative to pure phase space</li> | |
215 | <li> 42 - 50 : turn partons into a random number of hadrons, picked according | |
216 | to a Poissonian with average value as described above, but at least | |
217 | <code>code</code> - 40 and at most 10, and then distribute then in pure | |
218 | phase space; make a new try with another multiplicity if the sum of daughter | |
219 | masses exceed the mother one </li> | |
220 | <li> 52 - 60 : as 42 - 50, with multiplicity between <code>code</code> - 50 | |
221 | and 10, but avoid already explicitly listed non-partonic channels</li> | |
222 | <li> 62 - 70 : as 42 - 50, but fixed multiplicity <code>code</code> - 60</li> | |
223 | <li> 72 - 80 : as 42 - 50, but fixed multiplicity <code>code</code> - 70, | |
224 | and avoid already explicitly listed non-partonic channels</li> | |
225 | <li> 91 : decay to <ei>q qbar</ei> or <ei>g g</ei>, which should shower | |
226 | and hadronize</li> | |
227 | <li> 92 : decay onium to <ei>g g g</ei> or <ei>g g gamma</ei> | |
228 | (with matrix element), which should shower and hadronize</li> | |
229 | <li> 100 - : reserved for the description of partial widths of | |
230 | <aloc href="ResonanceDecays">resonances</aloc></li> | |
231 | </ul> | |
232 | ||
233 | Three special decay product identity codes are defined. | |
234 | <ul> | |
235 | <li>81: remnant flavour. Used for weak decays of c and b hadrons, where the | |
236 | c or b quark decays and the other quarks are considered as a spectator | |
237 | remnant in this decay. In practice only used for baryons with multiple | |
238 | c and b quarks, which presumably would never be used, but have simple | |
239 | (copied) just-in-case decay tables. Assumed to be last decay product.</li> | |
240 | <li>82: random flavour, picked by the standard fragmentation flavour | |
241 | machinery, used to start a sequence of hadrons, for matrix element | |
242 | codes in 41 - 80. Assumed to be first decay product, with -82 as second | |
243 | and last. Where multiplicity is free to be picked it is selected as for | |
244 | normal quarkonic systems. Currently unused.</li> | |
245 | <li>83: as for 82, with matched pair 83, -83 of decay products. The | |
246 | difference is that here the pair is supposed to come from a closed gluon | |
247 | loop (e.g. <ei>eta_c -> g g</ei>) and so have a somewhat higher average | |
248 | multiplicity than the simple string assumed for 82, see the | |
249 | <code>ParticleDecays:multGoffset</code> parameter above.</li> | |
250 | </ul> | |
251 | ||
252 | </chapter> | |
253 | ||
254 | <!-- Copyright (C) 2010 Torbjorn Sjostrand --> | |
255 |